RFR: 8342662: C2: Add new phase for backend-specific lowering [v2]

[Vladimir Ivanov]

On Mon, 28 Oct 2024 07:10:59 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

> The application of lowering is very broad as it can help us perform arbitrary transformation as well as take advantages of GVN 

@merykitty thanks for the examples. The idea of gradual IR lowering is not new in C2. There are precedents in the code base, so I'd like to better understand how the new pass improves the overall situation. Introducing a way to perform arbitrary platform-specific transformations on Ideal does sound very powerful, but it also turns Ideal IR into platform-specific dialects which don't have to work with existing transformations (IGVN, in particular).

Do the use cases mentioned so far justify a platform-specific lowering pass on Ideal IR which is intended to produce platform-specific Ideal IR shapes? I don't know yet.

Also, there are alternative places where platform-specific transformations can take place (macro expansion, final graph reshaping, custom matching logic). Worth considering them as well.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/21599#issuecomment-2442814693